Detection scheme for particle size and concentration measurement

ABSTRACT

The present invention provides a system and method of particle size and concentration measurement that comprises the steps of: providing a focused, synthesized, structured laser beam, causing the beam to interact with the particles, measuring the interaction signal and the number of interactions per unit time of the beam with the particles, and using algorithms to map the interaction signals to the particle size and the number of interactions per unit time to the concentration.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a Continuation of U.S. patent applicationSer. No. 15/440,287, filed Feb. 23, 2017; which application is aContinuation of U.S. patent application Ser. No. 14/359,233, filed onMay 19, 2014; which was a National Phase of PCT internationalapplication number PCT/IL2012/050488, having an international filingdate of Nov. 29, 2012, published as International Publication number WO2013/080209; which claimed priority and the benefit from U.S.provisional patent application No. 61/565,529, filed on Dec. 1, 2011;all of which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention is related to the field of measuring particle sizeand concentration. More specifically it relates to the use of opticalmethods for measuring particle size and concentration.

BACKGROUND OF THE INVENTION

Many techniques exist for particle size and concentration analysis(PSA), they can be reviewed for reference in the book by Terry Alan“Introduction to Particle Size Analysis” T. Allen, Particle sizeanalysis John Wiley & Sons; ISBN: 0471262218; June, 1983. The mostcommonly used techniques are optical, based on the interaction of themeasured particles with laser radiation. Especially when approaching theparticle size range around 1 micron and below, most of these techniquessuffer from inaccuracies due to the effect of the real and imaginarypart of the particle's refractive index. It is known, for example, thatin some techniques, such as techniques based on Fraunhoffer diffractionanalysis, light absorbing particles would be over sized due to energyloss resulting from the absorption, while in high concentration,particles would be under sized due to secondary scattering etc. Anoptical technique that is less sensitive to these problems is known asTime of Transition or TOT. In this technique the interaction of ascanning, focused laser beam and the particles is analyzed in the timedomain rather than in the amplitude domain, resulting in lowersensitivity to variation in the refractive index. A detailed descriptionof the technique appears in a paper “Improvements in Accuracy and SpeedUsing the Time-of-Transition Method and Dynamic Image Analysis ForParticle Sizing by Bruce Weiner, Walter Tscharnuter, and Nir Karasikov”,[Particle Size Distribution III; Assessment and Characterization;Editor(s): Theodore Provder1; Volume 693, Publication Date (Print): Jun.10, 1998; Copyright @ 1998 American Chemical Society]. To a greatextent, in this technique a de-convolution algorithm of the known laserbeam profile from the interaction signal derives the size. Theconcentration is derived from the number of interactions per unit timewithin the known volume of the focused laser beam.

The interaction of the particles in the TOT technique is with a focusedscanning laser beam. In order to measure smaller particles, a smallerfocused spot should be used. However according to diffraction laws for aGaussian laser beam, if the beam's waist is D, the divergence of thebeam is proportional to λ/D where λ is the laser's wavelength. Thetrade-off between the ability to resolve small particles, to the focusvolume and the accuracy in measuring concentration is obvious. Thus ifthe TOT technique is targeted to resolve and measure particles in themicron and sub-micron range it would be limited in its ability tomeasure low concentrations as the instantaneous focus volume is smalland the interaction rate of particles is low. On the other hand, takinga larger spot will improve the concentration measurement rate but willdegrade the quality and resolution of the size analysis.

An improvement could be achieved by using a shorter wavelength. Thiscould have a limited effect of as high as a factor of 2 only since goingto too short a wavelength will result in absorption of the laser lightby the optics and, in the case of particles in liquid, also absorptionby the liquid.

A previous invention by the inventors (U.S. Pat. No. 7,746,469)introduced a new technique and means to decouple between the twocontradicting requirements: the ability to resolve small particles andthe ability to measure low concentration using measurements based onsingle particle interactions using a structured laser beam.

It is therefore a purpose of the present invention to provide newdetection schemes offering higher sensitivity due to lower particlediameter dependency of the interaction signal.

It is another purpose of the present invention to provide new detectionschemes offering the ability to measure higher particle concentrationdue to inherent optical noise filtration.

It is another purpose of the present invention to provide new detectionschemes offering the ability to characterize particles by theirinteraction signal both in forward and in back scatter.

Further purposes and advantages of this invention will appear as thedescription proceeds.

SUMMARY OF THE INVENTION

The present invention provides a system and method of particle size andconcentration measurement that comprises the steps of: providing afocused, synthesized, structured laser beam, causing the beam tointeract with the particles, measuring the interaction signal and thenumber of interactions per unit time of the beam with the particles, andusing algorithms to map the interaction signals to the particle size andthe number of interactions per unit time to the concentration.

The particles can be fluid borne, airborne, or on a surface and have asize ranging from sub-micron to thousands of microns. In a preferredembodiment of the invention, the focused, synthesized, structured laserbeam is a dark beam.

The structured beam can be generated by employing a mask over a Gaussianlaser beam, by directly modifying the laser cavity, by combining thebeams from several lasers, or by other manipulations of the laser beamsuch as in an interferometric or polarization modification scheme. Themeasurements can be made using the duration of interaction with ascanning beam, including dark field. The invention further provides asystem for particle size and concentration measurement.

An alternative approach, which has the advantage of not using any movingparts to scan the beam, is to cause the particles to cross focal regionof a focused laser beam.

Other aspects of the invention relate to an improved detection schemecapable of better particle characterization according to the forward andback scatter, detect particle fluorescence and measure the particlevelocity.

The present invention introduces new detection schemes offering, highersensitivity due to lower particle diameter dependency of the interactionsignal (much lower than r{circumflex over ( )}4 to r{circumflex over( )}6 as with conventional scattering of sub wavelength particles); theability to measure higher particle concentration due to inherent opticalnoise filtration; the ability to characterize particles by theirinteraction signal in forward and back scatter, for example todiscriminate between bubbles and particles flowing in a liquid; theability to measure fluorescence from particles; and the ability tomeasure the particle's velocity. The latter enables a scanner freesystem where the flow of particles is either at a known velocity or thevelocity of each particle is intrinsically measured.

The invention is a particle monitoring system comprising a laser thatgenerates a Gaussian beam; means for converting the Gaussian laser beaminto a structured dark beam; a focusing lens that focuses the dark beamonto particles moving through the illuminating dark beam; and twodetectors. One of the two detectors is positioned over each intensitylobe of the dark beam.

The particle monitoring system of the invention is arranged such thatthe particles move through the illuminating dark beam in a direction atan angle of 90 degrees relative to the direction of the dark beam.

The signals from the two detectors are recorded in at least one of thefollowing ways:

-   -   a) as separate signals;    -   b) as a differential signal of the two detector signals; and    -   c) as the sum of the two detector signals.

Embodiments of the particle monitoring system of the invention comprisea beam splitter and a second set of detectors oriented in aperpendicular direction to the dark line of the dark beam.

Embodiments of the particle monitoring system of the invention comprisea beam splitter and a third detector arranged to allow simultaneousmeasurement of back scattered radiation from the particles.

All the above and other characteristics and advantages of the inventionwill be further understood through the following illustrative andnon-limitative description of embodiments thereof, with reference to theappended drawings. In the drawings the same numerals are sometimes usedto indicate the same elements in different drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an embodiment of a particle monitoringsystem;

FIG. 2 shows the positioning of the detectors of the system of FIG. 1with respect to the illuminating dark beam pattern;

FIG. 3(a) and FIG. 3(b) show typical signals measured by the twodetectors in the system of FIG. 1;

FIG. 4 is a scattering simulation showing half of the signals detectedby the two detectors of the system of FIG. 1 for air bubbles and latexparticles in water;

FIG. 5 shows simulated signals for the difference between the signalsfrom the two detectors of FIG. 1 as particles of various size move fromthe center outward;

FIG. 6 schematically shows an embodiment of the detector system of FIG.1 that has been modified to allow also the measurement of backscatteringof radiation from the particles;

FIG. 7 shows an example of how the invention can be used forclassification by clustering;

FIG. 8 shows an example of multi-dimensional clustering using anun-supervised learning method;

FIG. 9 schematically shows the profile of the dark beam;

FIG. 10A shows the differential signal of the two detector signals forthree illuminating beam structures in the presence of noise and theadvantage of the dark beam in suppressing common noise;

FIG. 10B shows the summation signal of the two detector signals for samethree illuminating beam structures as in FIG. 10A;

FIG. 11 is a screen shot showing two shoulders in the interactionsignal, which correspond to half the spot size; and

FIG. 12 shows the positioning of the detectors with respect to theilluminating dark beam pattern in an embodiment of the system of theinvention that comprises a second set of two forward detectors and abeam splitter oriented in a perpendicular direction to the dark line ofthe dark beam.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 schematically shows an embodiment of a particle monitoringsystem. The system shown in FIG. 1 comprises a laser (1), whichgenerates a Gaussian beam; collimating lens (2); phase mask (3), whichconverts the Gaussian laser beam into a structured dark beam; a focusinglens (4), which focuses the dark beam inside a cuvette (5) through whichwater containing particles (6) flows in the direction of the arrow; andtwo detectors (7) and (8). It is noted that in the case of airborneparticles, the air stream bearing the particles need not be confinedwithin a cuvette. The positioning of the detectors with respect to theilluminating dark beam pattern is shown in FIG. 2. In this embodimentone detector is positioned over each intensity lobe of the original darkbeam. As particles cross the beam the output intensity pattern ismodified and the detectors sense the change. The detector spacing can beoptimized for sensitivity by aligning it to the maximum intensitygradient of the dark beam. For various analytic purposes the detectorsignals can be recorded either as:

a) separate signals;b) as a differential signal of the two detector signals; andc) as the sum of the two detector signals.

In another embodiment, a second set of two forward detectors is used viaa beam splitter in a perpendicular direction to the dark line of thedark beam. These two detectors are large relative to the beam size andintegrate the total beam intensity. FIG. 12 shows the positioning of thesecond set of detectors with respect to the illuminating dark beampattern in this embodiment, By checking the symmetry of the signals fromthese two detectors, one can derive if a particle has crossed the focalzone along its diameter (signals are equal), or along a chord (signalsare different) as well as important information on the particle sizelike the width of the interaction or the depth of modulation in thesignal. The timing of these two signals provides information also on thealignment of the particle flow direction and to what extent it islaminar and perpendicular to the optical axis.

Typical signals measured by the two detectors in the system of FIG. 1are shown in FIG. 3(a) and FIG. 3(b). In these figures the signalsmeasured by detector 1 (7) are identified by numeral (10) and those bydetector 2 (8) are identified by numeral (12). For proper signalinterpretation it is necessary to confirm that the particle crossed thebeam along the focal plane. According to the present invention thesignals of the two detectors appear simultaneously when the particlecrosses the beam at focus, as shown in FIG. 3a . If the particle doesnot cross along the focal plane one detector signal is delayed withrespect to the other one as is shown in FIG. 3b . The shift directiondetermines also whether the particle crosses the beam before or afterthe focus. It should be noted that the shape of the signal representsintrinsic particle characteristics.

Since the detector signals qualitatively represent interferometricresponse they react to the phase modulation by the moving particles.Thus, a particle with refractive index larger than the surroundingmedium, e.g. latex in water, will induce, as an example, first anegative signal in detector 1 and a positive signal in detector 2 whilea particle with refractive index smaller than the medium (bubble inwater) will generate the opposite signal. It should be noted that themain feature is the opposing signals. By changing the detectors orderpositive/negative could be reversed. FIG. 4 shows half of the simulatedsignals in the forms of graphs showing detector output vs. the distancemoved by the particles for air bubbles and latex particles in water. Asa consequence, it is possible to distinguish bubbles from particles. Inthe figure, curve (14) shows the signal from sensor 1 for air in water;curve (16) shows the signal from sensor 2 for air in water; curve (18)shows the signal from sensor 1 for latex in water; and curve (20) showsthe signal from sensor 2 for latex in water.

Another important aspect of the present invention is a detection schemewith a lower signal intensity dependence on the particle radius r.According to the classical scattering theory, the energy scatteredbehaves according to r{circumflex over ( )}4 or even r{circumflex over( )}6 while in the present invention the signal is a result of a phaseshift and the r dependence is between r{circumflex over ( )}2 tor{circumflex over ( )}3. FIG. 5 is an example of a simulation showingthe dependence of the difference signal, i.e. the difference between thesignals of the two detectors for three particles of different size (r=50nm—dotted line; r=100 nm—solid line; r=200 nm—dashed line) moving outfrom the center of the dark beam pattern shown in FIG. 2. The advantageover the prior art provided by the invention is of a lower requireddetector dynamic range and a simpler detection scheme. The challenge ofhaving a detector with a dynamic range of 1:10,000 to 1:1,000,000, asneeded according to Rayleigh to detect particles in the range 0.1 to 1micron, is clear to anyone familiar with the art.

For particles typically larger than the spot size, the intensity on thetwo detectors will reach a plateau and the measuring parameter will bethe detectors summation width, which is proportional to the particlesize.

FIG. 6 schematically shows an embodiment of the detector system of FIG.1 that has been modified to allow measurement of backscattering ofradiation from the particles. The setup is as explained herein above forFIG. 1 with the addition of a beam splitter (24), collecting lens (26),pinhole (28), and the back scatter detector (30). The back scatterradiation from a particle (6) in the focus of the focusing lens (4) iscollected by the focusing lens (4), collimated, reflected by beamsplitter (24), and directed via the collecting lens (26), which focusesthe radiation through pinhole (28) onto the back scatter detector (30).In addition another lens (32) has been added, as may be needed by thetype of laser output, between the laser (1) and collimating lens (2)such that lens (32) and (2) act together as a beam expander (34).

In the present invention the back scatter detector has four roles:

-   -   The obvious use of back scatter detector (30) is in a confocal        detection scheme to verify that the interaction with the        particle was indeed in focus.    -   To provide additional size information, where, for particles        smaller than the dark spot, the dark beam modulation is        inversely proportional to the particle size. On the other hand,        for particles larger than the dark spot and moving with constant        velocity, the interaction duration is proportional to the        particle size.    -   The back scattering interaction adds another dimension for        differentiation among particle groups based on the fine details        of the interaction fingerprint, which could include reflection        properties.    -   The back scatter detector can detect fluorescence generated by        the illuminating beam. In this application the beam splitter        (24) is replaced by a dichroic mirror that would reflect the        fluorescent light to the BS detector. The ability to measure the        fluorescent light in parallel to the detection with the forward        detectors, adds a powerful classification tool in cases in which        the particle population was stained with a fluorescent stain.        This is extremely applicable to Algae to help characterize the        algae type or to the detection of pathogenic organisms.

A combination of a beam splitter and a dichroic mirror would allow todetect by two back scatter detectors the back scattered light and thefluorescent light.

The two forward signals and the optional back scatter signals (with andwithout Fluorescence) are single particle interactions with a highresolution laser focused beam. These interactions function as a highresolution one dimensional scanning laser microscope and provide a lotof information on the particle infrastructure. This information could beused to characterize specific particles. Particles of the same size butdifferent internal structure will have the same interaction width butthe internal interaction pulse behavior will differ and be like a“fingerprint” of the particle. An example of how the invention can beused for classification by clustering is shown in FIG. 7 for algae.

FIG. 7 shows the data in a two dimensional feature space. A set ofinteractions were acquired for 3 different types of Algae: Chlorela,Tetrahedron, and Pediastrum (indicated in the figure respectively by x,+, and *). Validation filters on the interaction signals, includingsymmetry of the two channels; interaction rise time, and others wereapplied. The Validation filters assure that the interaction is in thefocal zone of the dark beam. Tests were conducted with various sub setsof filters but FIG. 7 presents the data when all validation filters wereapplied. Features are extracted from the interactions that qualified thevalidation filters and show the clustering of the different types ofalgae.

Although the feature space is multi-dimensional, FIG. 7 presents a 2Dscatter diagram wherein the X axis is the interaction pulse width inmicroseconds and the Y axis the maximal signal per interaction asdetected by the detectors. Already in this 2D presentation there is aclear grouping of the different algae types. To help manifest thisgrouping ellipses were outlined in the figure to indicate the boundariesbetween the groups. In this 2D presentation there is still some overlap,which could be reduced in a multi-dimensional feature space. ArtificialIntelligence clustering techniques are then used, in the multidimensional space, to identify the boundaries.

The application of this mechanism is such that once the clusters ofknown algae are established, it is possible to monitor, for example,water contaminated with algae and detect in a mixture whether algaecomplying with the clusters appear. This would give real timeinformation on the algae population and feedback to any processattempting to reduce the algae population.

While the capability of the invention is demonstrated herein on algae,it can be used with all its detection options described above, to trainthe system on other events, such as pathogenic organisms and uponappearance of an event complying with the cluster of the said organisman alarm signal will be triggered. The analogy is drawn to afingerprint, where objects of the same group will have commonality inthe feature space and could be identified via this commonality in thesame way a person is identified by his fingerprint.

The classification by clustering approach can be extended to amulti-dimensional space using artificial intelligence tools to teach thesystem the nature of specific events and then to monitor for thepresence of such events. One embodiment of the invention is to clusterthe detectors signals by un-supervised learning (Visithttp://www.autonlab.org/tutorials/ for Andrew's repository of DataMining tutorials). FIG. 8 shows an example of multi-dimensionalclustering using an un-supervised learning method.

The measuring systems of FIG. 1 and FIG. 6 can be used to measure theintrinsic velocity of the particles moving through the cuvette. This ispossible because the interaction signal duration is scaled inversely tothe particles velocity. While in some configurations a constant andknown velocity could be achieved a more general approach of the presentinvention is to extract the velocity information from the intrinsicinformation in the interaction signal. This is done by accounting forthe profile (shown schematically in FIG. 9) of the dark beam. Thedistance between the intensity peaks of the lobes of the beam profile isequal to WO*2{circumflex over ( )}0.5, where W0 is the Gaussian waist.This value is known and hence can be used to measure the crossingparticle velocity, while the modulation depth is used to extract thesize information. This is applicable for small particles where the “lenseffect” of the particle is negligible. For larger particles there willbe 2 shoulders in the interaction signal which correspond to half thespot size, as shown in FIG. 11.

Many particle monitoring applications are characterized by a largepopulation of very small particles, doped with slightly largerparticles. Examples could be colloids; CMP slurry; crystallizationprocesses and more. The ratio of the tail concentration of the slightlylarger particles could be 10{circumflex over ( )}6:1 or so smallercompared to the main concentration. The state of the art instrumentationtoday is practically blind to these small concentrations. While smalland challenging to measure, this small tail could cause damages andscratches in the case of CMP or other processes. The present inventionoffers the ability to measure concentrations which are 10{circumflexover ( )}6:1 smaller in this tail. The spot size is selected such thatthe majority of the population is filtered out and becomes a backgroundnoise while the larger particles are shown as clear interactions.

The present invention, based on the dark beam illumination inconjunction with recording the detector signals as a differential signalof the two detectors is extremely robust to the background noise and canfacilitate detection in a high level of background noise. Thisrobustness is illustrated by the simulations presented in FIG. 10A andFIG. 10B. FIG. 10A shows the differential signal on an arbitrary scalewhile a 200 nm particle crosses the beam from the center for threeilluminating beam structures: Gaussian (dashed lines), Gauss-Laguerre(solid lines), and Dark beam (dotted lines). FIG. 10B is similar to FIG.10A but for the summation of the two detector signals. The simulationswere conducted under semi-dynamic noise containing 10% of the totalilluminating power and the other optical parameters in the simulationwere as follows:

-   -   NA=0.125    -   λ=400 nm

In FIG. 10A a signal higher by a factor of two is achieved for the darkbeam compared to the signals for the other two beam structures. Thedifference in sign of the curves is not material and depends on theorder of the detectors in the subtraction relative to the particledirection.

Bearing in mind the larger spot and depth of field of the dark beamcompared to the Gaussian beam, for achieving a valid measurement ofparticles smaller than spot, the noise immunity is even more apparent.The significant advantage in the noise reduction of the differentialconfiguration (FIG. 10A) as compared to the summation configuration(FIG. 10B) is obvious.

In actual measurements a measurement set-up based on the presentinvention was able to detect the tail of larger particles, theconcentration of which was 10{circumflex over ( )}6 times smaller inconcentration than the main population of the smaller particles.

Although embodiments of the invention have been described by way ofillustration, it will be understood that the invention may be carriedout with many variations, modifications, and adaptations, withoutexceeding the scope of the claims.

1-9. (canceled)
 10. A particle monitoring system comprising: a particleinterrogation region; a light source configured to produce a structuredlaser beam directed at the particle interrogation region, the structuredlaser beam having first and second intensity lobes; a first detectorpositioned to detect the first intensity lobe of the beam after the beamhas passed through the particle interrogation region; and a seconddetector, spaced apart from the first detector and positioned to detectthe second intensity lobe of the beam after the beam has passed throughthe interrogation region.
 11. The particle monitoring system of claim10, wherein the light source comprises a Gaussian laser beam.
 12. Theparticle monitoring system of claim 10, configured to record adifferential signal from the first and second detectors.
 13. Theparticle monitoring system of claim 10, configured to record a firstsignal from the first detector and a second signal from the seconddetector.
 14. The particle monitoring system of claim 10, configured torecord a summed signal from the first and second detectors.
 15. Theparticle monitoring system of claim 10, wherein the structured laserbeam is a structured dark beam.
 16. The particle monitoring system ofclaim 10, wherein the structured laser beam is configured such that afocal region of the structured laser beam occurs in the particleinterrogation region.
 17. The particle monitoring system of claim 10,wherein the particle interrogation region comprises a cuvette.
 18. Theparticle monitoring system of claim 10 comprising a processor configuredto carry out algorithms to analyze signals from the first and seconddetectors.
 19. The particle monitoring system of claim 10, comprising avalidation filter configured to validate that a particle interactionoccurred in a focal zone of the structured laser beam.
 20. A method ofparticle monitoring comprising: flowing particles through a particleinterrogation region of a particle monitoring system; producing astructured laser beam via a light source of the particle monitoringsystem, the structured laser beam having first and second intensitylobes; directing the structured laser beam at the particle interrogationregion; detecting the first intensity lobe of the beam after the beamhas passed through the particle interrogation region via a firstdetector; and detecting the second intensity lobe of the beam after thebeam has passed through the interrogation region via a second detector,the second detector spaced apart from the first detector.
 21. The methodof claim 20, comprising measuring a concentration of a subset ofparticles in a population of particles, via the particle monitoringsystem, wherein the particles of the subset are larger in size than therest of the population of particles.
 22. The method of claim 21, whereinthe step of measuring the concentration of the subset of particlescomprises configuring the structured laser beam such that a majority ofparticles of the population are filtered out by the particle monitoringsystem as background noise.
 23. The method of claim 22, wherein the stepof configuring the structured laser beam comprises tuning a spot size ofthe structured laser beam.
 24. The method of claim 21, wherein thesubset of particles comprises 1 one-millionth or less of the populationof particles.
 25. The method of claim 20, comprising analyzing signalsfrom the first and second detectors via a processor of the particlemonitoring system.
 26. The method of claim 25, wherein the particles aresuspended in a liquid, and wherein analyzing step comprisesdistinguishing particles from bubbles in the liquid.
 27. The method ofclaim 25, wherein the analyzing step comprises recording a differentialsignal from the first and second detectors.
 28. The method of claim 25,wherein the analyzing step comprises recording a first signal from thefirst detector and a second signal from the second detector.
 29. Themethod of claim 25, wherein the analyzing step comprises recording asummed signal from the first and second detectors.
 30. The method ofclaim 25, wherein the analyzing step comprises applying a validationfilter to validate that a particle interaction occurred in a focal zoneof the structured laser beam.
 31. The method of claim 25, wherein theanalyzing step comprises classifying the particles via artificialintelligence clustering.
 32. The method of claim 20, wherein the lightsource comprises a Gaussian laser beam.